1. Field of the Invention
This invention relates generally to fiber optic interferometer sensing systems that utilize the Sagnac interferometer having counterpropagating light beams, and more particularly to Sagnac interferometer sensing apparatus that utilize frequency shifters to provide a phase shift between the counterpropagating light beams to offset and null out nonreciprocal phase shifts induced by environmental factors.
2. Description of the Prior Art
In recent times the Sagnac interferometer utilizing counterpropagating light beams for sensing environmental factors that induce a nonreciprocal phase shift in the counterpropagating light beams has found various applications such as in acoustic sensors, temperature and pressure sensors, and more commonly optical gyros. Phase nulling fiber optic gyros, have been disclosed in the article, "Phase Nulling Fiber Optic Laser Gyro," Optic Letters, Volume 4, page 93, March, 1979, by R. F. Cahill and E. Udd; "Solid State Phase Nulling Optical Gyro," Applied Optics, Volume 19, page 3054, Sept. 15, 1980, by R. F. Cahill and E. Udd; and "Techniques for Shock-Noise-Limited-Inertia Rotation Measurement Using a Multiterm Fiber Sagnac Interferometer" by J. L. Davis and S. Ezekiel, Dec. 13, 1978, in SPIE, Volume 157. These papers disclose early fiber optic gyros which, although were known to operate, had not yet been involved in the intensive development required to bring a concept to practical use. Problems inherent in ring laser gyros were bypassed by adopting passive cavity techniques and applying fiber optics. Nonlinear analog outputs of prior art fiber optic gyros, which otherwise limit their dynamic range to a much lower level than that achieved by ring laser gyros are circumvented by using the phase nulling concept of Cahill and Udd disclosed in U.S. Pat. No. 4,299,490 for phase nulling optical gyro.